Porous media cold plate

ABSTRACT

A heat exchanger for cooling a heat generating device including a base having a recess with a base coolant inlet opening and a base coolant outlet opening. A porous core is positioned within the recess of the base, and has a core coolant inlet opening and a core coolant outlet opening that are arranged in corresponding relation with base coolant inlet opening and a base coolant outlet opening so as to be in fluid communication. A porous gasket is pinched between the porous core and the base.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/512,736, filed Oct. 20, 2003.

FIELD OF THE INVENTION

The present invention generally relates to heat exchanging devices andmore particularly, to devices adapted for removing heat from electroniccircuits, systems, and the like.

BACKGROUND OF THE INVENTION

It has been suggested that a computer is a thermodynamic engine thatsucks entropy out of data, turns that entropy into heat, and dumps theheat into the environment. The ability of prior art thermal managementtechnology to get that waste heat out of semiconductor circuits and intothe environment, at a reasonable cost, has limited the density and clockspeeds of electronic systems.

A typical characteristic of heat transfer devices for electronic systemsis that the atmosphere is the final heat sink. In heat transfer devicesfor electronics today, a semiconductor chip is often arranged so as tothermally contact a passive heat spreader plate, which conducts the heatfrom the chip to one of several types of fins. The fins, in turn,convect heat to the atmosphere with natural or forced air convection. Asthe power to be dissipated by semiconductor devices increases with time,a problem arises in that the thermal conductivity of the availablematerials becomes too low to conduct the heat from the semiconductordevices to fins with an acceptable temperature drop. The thermal powerdensity emerging from semiconductor devices today is so high that evensolid copper or silver spreader plates are not adequate.

Thermal energy can sometimes be transported by an intermediate loop ofrecirculating fluid. Heat from a hot object is conducted into a heattransfer fluid, the fluid is pumped by some means to a differentlocation, where the heat is conducted out of the fluid into a convectivefin and finally into the atmosphere. For example, U.S. Pat. Nos.5,125,451 and 5,274,920, issued to Matthews, provide a microscopiclaminar-flow heat exchanger for cooling a heat generating device, suchas a semiconductor integrated circuit. The heat exchanger consists of aplurality of thin plates which have been laminated together to form ablock. The plates consist of thin copper foil strips each having amicroscopic recessed portion etched into one face of the plate. Theserecessed portions are chemically etched to a shallow dimension, on theorder of fifty microns deep, prior to lamination. Either before or afterthe plates are laminated together, holes are cut through the plates atopposite sides of the recessed portions such that when the stack islaminated the holes align to form a pair of coolant distributionmanifolds. Each of the manifolds is essentially a tube which penetratesinto the block. The tubes are connected via the plurality of microscopicchannels formed from the recessed portions during the laminationprocess. Selectively adjusting the thickness of the channels and thecoolant flow rate allows the block to function as a heat exchanger. Asemiconductor die is simply placed or bonded onto the surface of theblock to effectuate heat removal.

A significant disadvantage with structures such as are taught byMatthews is the limited surface area available for coolant contact andconductive heat transfer. Additional available surface area or a moreserpentine coolant flow path, if provided, would greatly enhance theheat transfer characteristics of such devices. Unfortunately, themicroscopic size of Matthews' devices, and the etching techniques usedto manufacture such devices, do not provide for any meaningful increasein internal surface area or complex coolant flow paths, thereby limitingthe amount of thermal energy that can be removed by a single device.Furthermore, such structures do not lend themselves easily to the use ofinternal structures for the creation of turbulence in the coolant as itflows through the device.

Although the creation of turbulence in coolant as it flows through athermal transfer device is a well known technique for improving heattransfer, others have found that improved thermal performance can beachieved by configuring a fluid cooling device to support laminar fluidflow. For example, in U.S. Pat. No. 6,634,421, issued to Ognibene etal., a fluid cooling device is disclosed that includes a plurality ofcold plate members, each having a plurality of imperforate plateportions and perforate portions arranged in a line with at least oneconnector for connecting the plate portions together at one end. Thecold plate members are arranged in a stack, with respective plateportions of each cold plate member being in registration with perforateportions formed in its adjacent cold plate members in the stack. Thefluid cooling device appears to provide heat transfer by close clearancelaminar developing flow, which may increase the thermal performance ofthe fluid cooling device while maintaining low pressure drop.

None of the prior art has proved to be universally appropriate forachieving efficient thermal transfer in electronics systems.

SUMMARY OF THE INVENTION

The present invention provides a heat exchanger for cooling a heatgenerating device including a base having a recess with a base coolantinlet opening and a base coolant outlet opening. A porous core ispositioned within the recess of the base, and has a core coolant inletopening and a core coolant outlet opening that are arranged incorresponding relation with the base coolant inlet opening and a basecoolant outlet opening so as to be in fluid communication. A lid isfitted to the base so as to enclose the porous core, with a porousgasket pinched between the porous core and the lid.

In another embodiment of the invention, a heat exchanger for cooling aheat generating device is provided including a base having a recess witha coolant inlet opening and a coolant outlet opening. A complementarylid is sized so as to correspond to the size of the base, and has aninner surface with at least two fins that project outwardly from, andextend longitudinally along the inner surface in spaced apart relationto one another. Each of the fins is positioned within the recess andincludes a coolant inlet opening arranged in coaxial relation with thecoolant inlet opening of the base and a coolant outlet opening arrangedin coaxial relation with the coolant outlet opening of the base. Aporous core is positioned between the fins and within the recess, andincludes a core coolant inlet opening and a core coolant outlet opening.A metal felt gasket is pinched between the porous core and the base.

In a further embodiment of the invention, a pumped, single phase heatexchanger for cooling a heat generating device is provided including abase having a recess with a coolant inlet opening and a coolant outletopening. A lid that is complementarily sized and shaped includes aplurality of spaced-apart fins projecting outwardly from a surface andincludes a porous media positioned between adjacent ones of the fins soas to form porous cores. The fins and the cores include a core coolantinlet opening and a core coolant outlet opening, and are complementarilysized so as to be received within the recess. A porous gasket ispositioned within the recess and between the base and the fins and theporous media.

In yet another embodiment, a pumped, single phase heat exchanger forcooling a heat generating device is provided including a base having aperipheral wall that defines a recess. The peripheral wall also definesa first inlet opening and a first outlet opening that communicate withthe recess. A lid is provided having an inner surface that includes aplurality of spaced-apart fins that project outwardly from the innersurface. The fins comprise variations in height relative to the innersurface, and at least two fins also comprise variations in lengthrelative to others of the fins. A brazed porous media is positionedbetween adjacent ones of the fins so as to form a plurality of porouscores. The porous cores each comprise variations in height relative tothe inner surface and the fins. The brazed porous media and the finsfurther define a second inlet opening and a second outlet opening. Thefins and the plurality of porous cores are sized and shaped so as to becomplementary with the recess, and define an inlet plenum and an outletplenum. A porous and compressible gasket is positioned within the recessand between the base and the fins and the porous cores so as tocompressibly compensate for the variations in height.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bemore fully disclosed in, or rendered obvious by, the following detaileddescription of the preferred embodiments of the invention, which are tobe considered together with the accompanying drawings wherein likenumbers refer to like parts and further wherein:

FIG. 1 is a perspective view of a porous media cold plate formed inaccordance with the present invention;

FIG. 2 is an exploded perspective view of the porous media cold plateshown in FIG. 1;

FIG. 3 is a cross-sectional plan view of a base, as taken along line 3-3in FIG. 2;

FIG. 4 is a perspective view of a lid portion of a porous media coldplate formed in accordance with the present invention;

FIG. 5 is a bottom plan view of the lid shown in FIG. 4;

FIG. 6 is a cross-sectional view of the lid shown in FIGS. 4 and 5, astaken along line 6-6 in FIG. 5;

FIG. 7 is a cross-sectional view, not to scale, of a porous media coldplate formed in accordance with the present invention, as taken alongline 7-7 in FIG. 1, showing a schematic representation of a typicalcoolant flow pattern;

FIG. 8 is a cross-sectional view of the porous media cold plate, astaken along line 8-8 in FIG. 1;

FIG. 9 is an expanded enlarged view of a portion of the porous mediacold plate shown in FIG. 8;

FIGS. 10 and 11 are a cross-sectional view of a porous media cold plateformed in accordance with an alternative embodiment of the presentinvention, as taken along line 7-7 in FIG. 1, including an expandedenlarged view of a portion of the alternative embodiment of porous mediacold plate shown in FIG. 10; and

FIGS. 12 and 13 are graphical illustrations of the thermal performanceof the present invention in comparison to prior art cold plates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This description of preferred embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description of this invention. The drawingfigures are not necessarily to scale and certain features of theinvention may be shown exaggerated in scale or in somewhat schematicform in the interest of clarity and conciseness. In the description,relative terms such as “horizontal,” “vertical,” “up,” “down,” “top” and“bottom” as well as derivatives thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing figure underdiscussion. These relative terms are for convenience of description andnormally are not intended to require a particular orientation. Termsincluding “inwardly” versus “outwardly,” “longitudinal” versus “lateral”and the like are to be interpreted relative to one another or relativeto an axis of elongation, or an axis or center of rotation, asappropriate. Terms concerning attachments, coupling and the like, suchas “connected” and “interconnected,” refer to a relationship whereinstructures are secured or attached to one another either directly orindirectly through intervening structures, as well as both movable orrigid attachments or relationships, unless expressly describedotherwise. The term “operatively connected” is such an attachment,coupling or connection that allows the pertinent structures to operateas intended by virtue of that relationship. In the claims,means-plus-function clauses are intended to cover the structuresdescribed, suggested, or rendered obvious by the written description ordrawings for performing the recited function, including not onlystructural equivalents but also equivalent structures.

Referring to FIGS. 1-3, a porous media cold plate 2 formed in accordancewith the present invention includes a base 5, a lid 8, two or more platefins 10, a gasket 12, and a porous media 14. More particularly, base 5is often formed as a rectilinear enclosure having a bottom wall 22comprising a substantially uniform thickness of a thermally conductivematerial, e.g., copper, steel, aluminum, or any of their respectivealloys. Bottom wall 22 preferably comprises a substantially planer innersurface 24 and a peripheral edge wall 26. Peripheral edge wall 26projects outwardly from the peripheral edge of inner surface 24 so as tocircumscribe inner surface 24, thus defining a recessed region. An inletconduit 27 and an outlet conduit 29 are defined through portions ofperipheral edge wall 26 or, through bottom wall 22 or lid 8, and arearranged in fluid flow communication with one another. Gasket 12 ispositioned atop inner surface 24 of base 5, and preferably comprises ametal felt or metal wool. Although nonmetal porous gaskets may also beused with the present invention, they are not preferred. Advantageously,a metal felt or wool gasket 12 is both porous and compressible and isalso easily susceptible to brazing. This allows for brazed attachmentand little or no thermal expansion mismatches when positioned withinbase 5.

Lid 8 is sized so as to correspond to the size of base 5, and comprisesa substantially planar plate of thermally conductive material having aninner surface 30 and a peripheral edge surface 32. Two or more platefins 10 project outwardly from, and extend longitudinally along thelength of inner surface 30 in spaced apart relation to one another. Inone embodiment of the invention, outer most plate fins 10 a and 10 b arelonger at one end so as to define an inlet plenum 33 and an outletplenum 34 (FIG. 7) when lid 8 is assembled to base 5, as willhereinafter be disclosed in further detail. Each plate fin 10 includesan elongate top surface 35. It is very often the case that plate fins 10comprise a height above inner surface 30 that varies from fin to fin dueto normal dimensional tolerance variations (FIG. 9). A plurality ofcoaxial inlet openings 40 and coaxial outlet openings 42 are formedthrough several of plate fins 10. Of course, plate fins 10 may bepositioned on inner surface 24 of base 5 with adequate results.

In one embodiment, porous media 14 is located on inner surface 30 andbetween adjacent plate fins 10 so as to form a plurality of heattransfer columns or cores 44 that comprise a plurality of particles 45.Plurality of particles 45 are often combined with a filler metal orcombination of metals that is often referred to as a “braze” or brazingcompound, or even a solder. The porous media acts to provide forenhanced thermal communication between plate fins 10 and a coolant fluidflowing through porous media cold plate 2 (FIG. 7) by effectivelyextending the surface area of the fins. It will be understood that“brazing” is the joining of metals through the use of heat and a fillermetal, i.e., a brazing compound. Of course, a solder having comparableattributes may also be used with adequate results. A typical brazingcompound of the type used in the construction of the present inventionvery often comprises a melting temperature that is above 450° C.-1000°C., but below the melting point of particles 45 that are being joined toform porous media 14. Each core 44 of brazed particles 45 define anelongate, uneven top surface 47 that is substantially coextensive withtop surface 35 of each plate fin 10. Of course, cores 44 may also beformed from a variety of porous media 60, e.g., open cell foam, felt,wool, screen, or chopped wire (FIGS. 10 and 11). All of these porousmedia may be made from either metals or non-metals, and may be bonded tojoin them to a wall through which heat is transferred—this is importantfor using the additional surface area for heat transfer to a coolantfluid.

Particles 45 are very often formed from a conductive metal, e.g.,copper, but may be selected from any of the materials having highthermal conductivity, that are suitable for fabrication into brazedporous structures, e.g., carbon, tungsten, copper, aluminum, magnesium,nickel, gold, silver, aluminum oxide, beryllium oxide, or the like, andmay comprise either substantially spherical, oblate or prolatespheroids, and less preferably, arbitrary or regular polygonal, orfilament-shaped particles of varying cross-sectional shape. For example,when particles 45 are formed from copper spheres or spheroids (FIG. 9)whose melting point is about 1981° F./1083° C., the overall processingtemperature for porous media 14 will be about 1000° C.

In general, to form porous media 14 according to the present invention,a plurality of particles 45 and a suitable brazing compound, e.g., onecontaining about thirty-five percent gold and about sixty-five percentcopper, are heated together to a brazing temperature that melts thebrazing compound, but does not melt plurality of particles 45.Significantly, during brazing, particles 45 are not fused together aswith sintering, but instead are joined together by creating ametallurgical bond between the brazing compound and the surfaces ofadjacent particles 45 through the creation of fillets of re-solidifiedbrazing compound. Thus, as the brazing compound liquefies, the moleculesof molten brazing metals attract one another as the surface tensionbetween the molten braze and the surfaces of individual particles 45tend to draw the molten braze toward each location where adjacentparticles 45 are in contact with one another. Fillets are formed at eachsuch location as the molten braze metals re-solidify. The resultingstructure of each core 44 provides a porous labyrinth through whichcoolant fluid may flow and thereby experience increased heat transfer(FIGS. 7-10).

The enhanced thermal conductivity of porous media 14 directly improvesthe thermal conductance of the heat transfer device in which it isformed. Depending upon the regime of heat flux that porous media coldplate 2 is subjected to, the thermal conductance of brazed porous media14 has been found to increase as shown in FIG. 12, and FIG. 13represents hydraulic perfornmance.

Referring to FIGS. 1 and 2, a porous media cold plate 2 may be formed inaccordance with the present invention by brazing a plurality ofparticles 45 within the spaces between adjacent plate fins 10 on innersurface 30 of lid 8. It will be noted that after brazing, top surface 47of each column of brazed particles 45 is uneven with each having adissimilar height above inner surface 30 of lid 8 (FIG. 9). In addition,top surface 35 of each plate fin 10 will have a slightly differentheight above inner surface 30 of lid 8 with respect to one another and,also with respect to top surface 47 of each core 44.

With particles 45 brazed in cores 44 between adjacent plate fins 10, lid8 is positioned above base 5 so that peripheral edge surface 32 isarranged in spaced confronting relation to the top surface of peripheraledge wall 26 of base 5. Once in this position, lid 8 is moved towardbase 5 until brazed particles 45 and plate fins 10 engage metal feltgasket 12 located on inner surface 24 of base 5. As this occurs, inletopenings 40 are located in substantially aligned, often coaxial relationwith inlet conduit 27 and outlet openings 42 are arranged in alignedcoaxial relation with outlet conduit 29. Also, an end of each plate fin10 a and 10 b is located adjacent to a portion of peripheral edge wall26 so as to stop coolant flow and thereby define inlet plenum 33 andoutlet plenum 34 when lid 8 is assembled to base 5. Advantageously,inlet plenum 33 and outlet plenum 34 are sized and shaped so thatcoolant fluid entering porous media cold plate 2 via inlet conduit 27suffers little decrease or increase in velocity, i.e., thecross-sectional area of inlet plenum 33 and outlet plenum 34 is oftenapproximately equal to the cross-sectional area of inlet opening 27 andoutlet opening 42 (FIG. 7). Lid 8 is then brazed or welded to base 5along the interface between the top surface of peripheral edge wall 26and peripheral edge surface 32. At the same time, top surfaces 35 ofplate fins 10 and top surfaces 47 of cores 44 engage and compress metalfelt gasket 12 so that metal felt gasket 12 is pinched between these twostructures. It is often the case that gasket 12 comprises a more coarserporosity than porous media 14 or porous media 60 so as to enhancecompression and provide for some fluid flow when compressed by cores 44and fins 10.

During the brazing operation, plate fins 10 and cores 44 are brazed tometal felt gasket 12. It will be understood that a metal felt gasket mayalso be adhered to base 5 via solder or even thermal epoxy.Advantageously, the dimensional mismatch and unevenness between topsurfaces 35 and 47 are fully compensated by the compressibility of metalfelt gasket 12 while still allowing some fluid flow through the gasket.In addition, metal felt gasket 12 may be evenly brazed to both topsurface 35 and top surface 47, but due to its porosity, allows forresidual brazed materials and other bi-products to be flushed from eachcore 44 so as to further enhance their fluid flow characteristics andtherefore heat transfer capability. It should be noted that gasket 12starts out more porous, but after compression is less porous. It isdesirable that gasket 12 eliminate, or at least limit leakage flowbetween fins 10 and lid 8.

In operation, a coolant fluid is pumped through inlet conduit 27 andthereby into inlet plenum 33 and plurality of inlet openings 40 so as toflood each core 44. A portion of the coolant fluid also floods inletplenum 33 that surrounds the inlet portions of plate fins 10 and cores44 (FIG. 7). As the coolant fluid moves through porous media 14 of core44, heat is transferred to the fluid. The coolant fluid exits porousmedia cold plate 2 through outlet openings 42, outlet plenum 34, andoutlet conduit 29. By flowing single-phase coolant fluid through porousmedia 14, high effective heat transfer rates can be achieved atcomparatively low temperature differences. In order to mitigate anypressure differential required to drive the single phase coolant fluidthrough porous media 14, inlet plenum 33 and outlet plenum 34 provide ascheme which reduces the coolant mass flux and effective flow length.Cores 44 manifold the flow in such a way that the total cold plate massflow is subdivided into three streams, each of which flows through asegment of porous media 14 that is slightly less than one third of thetotal length of core 44. Cores 44 are bonded into base 5 which directsthe flow of coolant fluid to and from cores 44. Mounting features may beprovided for attaching cores 44, via an outer surface of lid 8, to aheat-generating device, e.g., a semiconductor device 100.

It is to be understood that the present invention is by no means limitedonly to the particular constructions herein disclosed and shown in thedrawings, but also comprises any modifications or equivalents within thescope of the claims.

1. A heat exchanger for cooling a heat generating device comprising: abase having a recess with a base coolant inlet opening and a basecoolant outlet opening; a porous core positioned within said recess andhaving a core coolant inlet opening and a core coolant outlet opening alid fitted to said base and enclosing said porous core; an inlet plenumdefined between said lid, said base, and said porous core, and an outletplenum defined between said lid, said base, and said porous core whereinsaid inlet plenum and said outlet plenum are in fluid communicationthrough said porous core; and a porous gasket pinched between saidporous core and said lid.
 2. A heat exchanger according to claim 1wherein said porous gasket comprises felt.
 3. A heat exchanger accordingto claim 1 wherein said porous gasket comprises metal wool.
 4. A heatexchanger according to claim 1 wherein said porous gasket is positionedatop an inner surface of said base, and comprises a compressible metalfelt.
 5. A heat exchanger according to claim 1 wherein said porousgasket is brazed to said base.
 6. A heat exchanger according to claim 1comprising a lid sized so as to correspond to the size of said base, andhaving an inner surface and a peripheral edge surface.
 7. A heatexchanger according to claim 6 wherein said lid includes at least twofins that project outwardly from, and extend longitudinally along saidinner surface in spaced apart relation to one another, wherein saidporous core is positioned between said fins.
 8. A heat exchangeraccording to claim 6 wherein said lid includes at a plurality of finsthat project outwardly from, and extend longitudinally along said innersurface in spaced apart relation to one another, wherein said porouscore is positioned between said fins and further wherein two outer mostfins of said plurality of fins have an end that is adjacent to a wallportion of said base so as to define said inlet plenum and said outletplenum.
 9. A heat exchanger according to claim 7 wherein each of saidfins includes a coolant inlet opening and a coolant outlet opening thatare spaced apart by a distance that is about one-third the total lengthof said porous core.
 10. A heat exchanger according to claim 7 whereineach of said fins includes an elongate top surface that has a heightabove said inner surface that varies from fin to fin.
 11. A heatexchanger according to claim 1 wherein said base coolant inlet openingis coaxial with said core coolant inlet openings, and said base coolantoutlet opening is coaxial with said core outlet openings.
 12. A heatexchanger according to claim 7 wherein said porous media is located onsaid inner surface and between adjacent fins so as to form a pluralityof heat transfer cores that comprise a plurality of brazed particles.13. A heat exchanger according to claim 12 wherein said plurality ofheat transfer cores define an elongate, uneven top surface that issubstantially coextensive with a top surface of each of said fins.
 14. Aheat exchanger according to claim 12 wherein said particles are selectedfrom the group consisting of carbon, tungsten, copper, aluminum,magnesium, nickel, gold, silver, aluminum oxide, or beryllium oxide. 15.A heat exchanger according to claim 12 wherein the shape of saidparticles is selected from the group consisting of substantiallyspherical, oblate, prolate spheroids, regular polygonal, orfilament-shaped.
 16. A heat exchanger for cooling a heat generatingdevice comprising: a base having a recess with a base coolant inletopening and a base coolant outlet opening; a porous core positionedwithin said recess and having a core coolant inlet opening, a corecoolant outlet opening and defining an inlet plenum and an outlet plenumthat are located between said base and said porous core; a lid fitted tosaid base and enclosing said porous core; and a porous gasket pinchedbetween said porous core and said lid.
 17. A heat exchanger for coolinga heat generating device comprising: a base having a recess with a basecoolant inlet opening and a base coolant outlet opening; a lid sized soas to correspond to the size of said base, and having an inner surfacewith a plurality of fins that project outwardly from, and extendlongitudinally along said inner surface in spaced apart relation to oneanother, each of said fins positioned within said recess and including acoolant inlet opening arranged in coaxial relation with said basecoolant inlet opening and a coolant outlet opening arranged in coaxialrelation with said base coolant outlet opening, wherein two outer mostfins of said plurality of fins have an end that is adjacent to a wallportion of said base so as to define said inlet plenum and said outletplenum; a porous core positioned between said fins and within saidrecess and having a core coolant inlet opening and a core coolant outletopening; and a metal felt gasket pinched between said porous core andsaid base.
 18. A pumped, single phase heat exchanger for cooling a heatgenerating device comprising: a base having a recess with a base coolantinlet opening and a base coolant outlet opening; a lid including aplurality of spaced-apart fins projecting outwardly from a surface andincluding a porous media positioned between adjacent ones of said finsso as to form a core, wherein said fins and said cores include a corecoolant inlet opening and a core coolant outlet opening and wherein twoouter most fins of said plurality of fins have an end that is adjacentto a wall portion of said base so as to define said inlet plenum andsaid outlet plenum that are located between said base and said porouscore and fins; and a porous gasket positioned within said recess andbetween said base and said fins and said porous media.
 19. A heatexchanger according to claim 18 wherein said porous gasket comprises acompressible metal felt.
 20. A heat exchanger according to claim 18wherein said porous gasket comprises a compressible metal wool.
 21. Aheat exchanger according to claim 18 wherein said porous gasket ispositioned atop an inner surface of said base, and comprises a metalfelt.
 22. A heat exchanger according to claim 18 wherein said porousgasket is brazed to said base.
 23. A heat exchanger according to claim18 comprising a lid sized so as to correspond to the size of said base,and having an inner surface and a peripheral edge surface.
 24. A heatexchanger according to claim 18 wherein each of said fins includes anelongate top surface that has a height above said inner surface thatvaries from fin to fin.
 25. A heat exchanger according to claim 18wherein said base coolant inlet opening is coaxial with said corecoolant inlet openings, and said base coolant outlet opening is coaxialwith said core outlet openings.
 26. A heat exchanger according to claim18 wherein said porous media comprises a plurality of brazed particles,and is located on an inner surface of said lid between adjacent fins soas to form a plurality of heat transfer cores.
 27. A heat exchangeraccording to claim 26 wherein said plurality of heat transfer coresdefine an elongate, uneven top surface that is substantially coextensivewith a top surface of each of said fins.
 28. A heat exchanger accordingto claim 26 wherein said particles are selected from the groupconsisting of carbon, tungsten, copper, aluminum, magnesium, nickel,gold, silver, aluminum oxide, or beryllium oxide.
 29. A heat exchangeraccording to claim 26 wherein the shape of said particles is selectedfrom the group consisting of substantially spherical, oblate, prolatespheroids, regular polygonal, or filament-shaped.
 30. A pumped, singlephase heat exchanger for cooling a heat generating device comprising: abase having a peripheral wall that defines a recess, said peripheralwall further defining first inlet opening and a first outlet openingthat communicate with said recess; a lid having an inner surfaceincluding a plurality of spaced-apart fins projecting outwardly fromsaid inner surface wherein said fins comprise variations in heightrelative to said inner surface and further wherein spaced apart outerones of said fins abut a portion of said base so as to define an inletplenum and an outlet plenum that are located between said base and saidfins; a brazed porous media positioned between adjacent ones of saidfins so as to form a plurality of porous cores, wherein said porouscores comprise variations in height relative to said inner surface andsaid fins, said brazed porous media and said fins further defining asecond inlet opening and a second outlet opening, and further whereinsaid fins and said plurality of porous cores are sized and shaped so asto be complementary with said recess; and a porous and compressiblegasket positioned within said recess and between said base and said finsand said porous cores, so as to compressibly compensate for saidvariations in height.